1. Field of the Invention
The present invention relates generally to light emitting diodes (LEDs) and, more particularly, to LEDs having nanoparticles and light scattering particles that improve the light extraction efficiency and spatial color temperature uniformity of the LED device.
2. Description of the Related Art
Solid state light emitting devices, such as inorganic or organic light emitting diodes (LEDs), convert energy to light and are widely used for many applications. As known to those having skill in the art, inorganic solid state devices generally include one or more active regions of semiconductor material interposed between oppositely doped regions. When a bias is applied across the doped regions, electron-hole recombination events occur that generate light, and light is emitted from the active region in omnidirectional paths from all surfaces of the LED. Conventional LEDs may incorporate reflectors and/or mirror surfaces to direct the emitted light in a desired direction.
The color or wavelength emitted by an LED is largely dependent on the properties of the material from which it is generated that determine the bandgap of the active region. LEDs have been built to emit light in a range of colors in the visible spectrum including red, yellow, green, and blue. Other LEDs emit radiation outside the visible spectrum such as in the ultraviolet (UV) range of the electromagnetic spectrum. It is often desirable to incorporate phosphors into a LED to tailor the emission spectrum by converting a portion of the light from the LED before it is emitted. For example, in some blue LEDs a portion of the blue light is “downconverted” to yellow light. Thus, the LED emits a combination of blue and yellow light to generate a spectrum that appears white to the human eye. As used herein, the term “phosphor” is used generically to indicate any photoluminescent material.
Phosphors have been disposed in various regions within the LED structure. For example, phosphor may be dispersed inside and/or coated outside a dome-shaped encapsulant that covers the device. The phosphor may be located remotely from the light emitting die as shown in U.S. Pat. No. 7,286,926. The phosphor may also be coated or deposited on the die itself. Several techniques are frequently used to introduce the phosphor, including electrophoretic deposition, stencil printing, spin or spray coating, etc. Another technique uses a phosphor dispense process where a drop of material, such as epoxy, silicone encapsulant, etc., that contains phosphor therein, may be placed on the die and cured to form a shell over the die. This is sometimes is sometimes referred to as a “glob top” process. In another technique, the drop of material that contains phosphor may be placed above the die, and the phosphor is allowed to settle within the drop. This technique may be referred to as “remote settling”.
Many applications require an LED that emits white light. As used herein, the term “white light” is used in a general sense and includes light that different individuals or detectors may perceive as having a slight tint toward, for example, yellow or blue. As discussed above, some conventional LED devices combine a yellow phosphor on a blue LED to achieve white light. Some of the blue light emitted from the LED passes through the phosphor without being converted, and some of the emitted blue light is downconverted to yellow. The combinations of blue light and yellow light that escape the light emitting device provide a white light output.
LEDs have been manufactured that include several other functional features, such as reflective/refractive layers, lenses and light scattering elements, for example. Some LEDs include surfaces that have been textured to enhance light extraction by reducing total internal reflection at various material interfaces. Many other functional features known in the art may be combined to build an LED having particular characteristics.
FIG. 1 shows a cross-sectional view of known LED device 10. An LED chip 11 is disposed on a mount surface 12. A layer of wavelength conversion material 13 surrounds the LED chip 11. An encapsulant 14 covers the LED chip 11 and the conversion layer 13. The LED chip has a textured emission surface 15 that helps to extract light at the interface of the LED chip 11 and the conversion layer 13 by countering the effects of the step-down in index of refraction at the interface. Phosphor particles 16 are shown dispersed throughout the conversion layer 13. Some of the light emitted from the LED chip 11 is reflected or backscattered inside the conversion layer 13, at the interface with encapsulant 14, or within the encapsulant 14 back towards the textured surface 12. Due to the textured surface 15 coupled with a step-up in index of refraction at the interface, a substantial portion of the light incident on the textured surface 15 re-enters the LED chip 11 where it may be absorbed, decreasing the light extraction efficiency of the device 10.